In an internal combustion engine for a vehicle, it is desirable to design an air induction system in which sound energy generation is minimized. Sound energy is generated as air is drawn into the engine. Vibration is caused by the intake air in the air feed line which creates undesirable intake noise. Resonators of various types such as a Helmholtz type, for example, have been employed to reduce engine intake noise by reflecting sound waves generated by the engine 180 degrees out of phase. The combination of the sound waves generated by the engine with the out of phase sound waves results in a reduction or cancellation of the amplitude of the sound waves. Such resonators typically include a single, fixed volume chamber for dissipating the intake noise. Multiple resonators are frequently required to attenuate several sound waves of different frequencies.
Desired noise level targets have been developed for a vehicle engine induction system. The noise level targets often cannot be met with a conventional multi-resonator system. The typical reason is that conventional resonator systems provide an attenuation profile that does not match the profile of the noise targets and yields unwanted accompanying side band amplification. This is particularly true for a wide band noise peak. The result is that when a peak value is reduced to the noise level target line at a given engine speed, the amplitudes of adjacent speeds are higher than the target line. Thus, the resonators are effective at attenuating noise at certain engine speeds, but ineffective at attenuating the noise at other engine speeds.
Existing controlled variable tuned resonators vary resonator volume to achieve the desired noise reduction as a function of engine speed. Volume control of the resonators requires the movement of large sealed areas, which presents several problems, including increased motor load and undesirable wear on the seal.
It would be desirable to produce a resonator that does not require sealing of the resonator volume and is variable tuned to militate against the emission of sound energy caused by the vehicle engine induction process at a wide range of engine speeds.